1. Field of the Invention
The present invention generally relates to displacement detecting apparatus and, more particularly, to a displacement detecting apparatus for detecting a displacement between an objective lens and a measured surface by using an optical astigmatism method, for example, and a displacement detecting apparatus for detecting a displacement of a measured surface by using a non-contacting sensor.
2. Description of the Related Art
Displacement detecting apparatus using a laser light, an objective lens or the like are known to measure a displacement or shape of a measured object.
This type of displacement detecting apparatus converges a laser light on a measured surface by an objective lens, obtains a focusing error signal from a reflected light of the laser light on the basis of an astigmatism method and then calculates a displacement of the measured surface from an absolute level (e.g., voltage value) of the focusing error signal.
As other conventional displacement detecting apparatus, there is known a displacement detecting apparatus in which a displacement of a measured surface is detected by using an interferometer or electric micrometer.
In the former conventional displacement detecting apparatus which calculates the displacement of the measured surface from the absolute level of the focusing error signal, a linearity of the focusing error signal itself is insufficient and hence detection accuracy cannot be increased without difficulty. Further, in this type of displacement detecting apparatus, a sensitivity (voltage/displacement) of the focusing error signal is changed with a reflection factor of the measured surface. Then, from this standpoint, it is impossible to increase a measurement accuracy. Furthermore, according to this type of displacement detecting apparatus, a range in which the focusing error signal is detected is small. There is then the disadvantage that a range in which a displacement of the measured surface is detected is unavoidably narrow.
In the latter conventional displacement detecting apparatus which can detect the displacement of the measured surface by using the interferometer or electric micrometer, the measurement work is cumbersome and the apparatus itself is expensive, large in size and low in stability or the like.
Further displacement detecting apparatus having a non-contacting sensor are known to detect a displacement or shape of a measured object.
This type of conventional displacement detecting apparatus effectively utilizes a detection optical system for generating a laser light and a focusing error signal and an objective lens as the non-contacting sensor, converges the laser light on a measured surface by the objective lens, obtains a focusing error signal from the reflected light of the laser light on the basis of the astigmatism method and then detect a displacement of the measured surface from the absolute level (e.g., voltage value) of the focusing error signal.
In the above displacement detecting apparatus for detecting the displacement from the absolute level of the focusing error signal, however, a linearity of the focusing error signal itself is insufficient so that a detection accuracy cannot be increased.
In order to enhance the accuracy with which the displacement is detected, it is proposed that a servo is effected on the displacement detecting apparatus so as to enable the non-contacting sensor to produce an output of a constant value. Then, the movement of the objective lens can be detected by reading a graduation of a linear scale which is designed so as to move in unison with the objective lens.
The above-mentioned displacement detecting apparatus in which the servo is effected to detect the displacement amount of the measured surface is not limited to the above technique which utilizes the detection optical system and a variety of techniques, such as a combination of a non-contacting sensor of a capacitance type and a linear scale, a combination of a scanning tunneling microscope (i.e., STM) and an interferometer or the like may be enumerated.
According to the conventional displacement detecting apparatus in which the detection accuracy can be increased by the servo control, however, because of the servo control, a displacement of a vibration in which a frequency characteristic is relatively low, for example, about 100 Hz can be detected at best. In order to increase the speed of the detection and measurement, the non-contacting sensor must not be servo-controlled and the output level thereof must be measured. If so, then the linearity is deteriorated and the detection accuracy also is lowered. As a result, the above-mentioned problems cannot be solved.